1. Field of the Invention
The invention pertains to a displacement measuring system for a piston-cylinder assembly including a cylinder, a piston rod guided for movement axially within the cylinder, a first position measuring device which moves with the piston rod, and a second position measuring device fixed to the cylinder.
2. Description of the Related Art
A sensor for a piston-cylinder assembly is known from DE 29 45 895 A1. The sensor is located inside a piston rod guide so that it can detect the stroke position of the piston-cylinder assembly at any specific moment. A general problem is that the piston rod can twist during operation. The only remedy here is either a piston rod which is “prepared” around its entire circumference or a sensor in the form of a circular ring.
DE 94 12 435 U1 describes a working cylinder, which has a piston rod with a magnetic strip inside it. The piston rod also has a rectangular, preferably square, cross section and cooperates with a corresponding square piston rod guide. The rod is thus prevented from twisting, which makes it possible to use a combination of a comparatively narrow magnetic strip and a sensor with a narrow measuring field, but the sealing of a square rod represents a much more difficult problem than the sealing of a uniformly round piston rod, especially when the square rod is to be installed in a mass-produced product and corresponding standard components and tools are to be used.